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Administrative Distance (AD) and Autonomous System (AS)
Administrative Distance (AD) and Autonomous System (AS) are fundamental concepts in computer networking that help manage routing decisions and network organization. Administrative Distance determines the trustworthiness of routing information, while Autonomous Systems define administrative boundaries in large networks.
What is Administrative Distance (AD)?
Administrative Distance is a trustworthiness metric used by routers to choose between routing information received from different sources. It ranges from 0 to 255, where lower values indicate higher trustworthiness.
When a router receives multiple routes to the same destination from different routing protocols, it uses AD to determine which route to install in the routing table. The route with the lowest AD value is preferred, regardless of other metrics like hop count or bandwidth.
Common AD Values
| Route Source | Default AD | Description |
|---|---|---|
| Connected Interface | 0 | Directly connected networks |
| Static Route | 1 | Manually configured routes |
| OSPF | 110 | Open Shortest Path First |
| RIP | 120 | Routing Information Protocol |
What is Autonomous System (AS)?
An Autonomous System is a collection of IP networks and routers under the control of a single administrative entity that presents a common routing policy to the Internet. Each AS is identified by a unique Autonomous System Number (ASN).
ASNs are essential for Border Gateway Protocol (BGP), which routes traffic between different autonomous systems on the Internet. Originally 16-bit numbers (1-65535), ASNs have been expanded to 32-bit to accommodate growth.
Key Differences
| Aspect | Administrative Distance | Autonomous System |
|---|---|---|
| Purpose | Route selection within a router | Network organization and BGP routing |
| Scope | Local to individual router | Global Internet routing |
| Range | 0-255 | 1-4,294,967,295 (32-bit) |
Practical Example
Consider a corporate network (AS 65001) connected to two ISPs. The border router receives routes to the same destination via OSPF (AD 110) internally and BGP (AD 20) externally. The router selects the BGP route due to its lower AD value, ensuring external traffic uses the ISP connection while internal traffic uses OSPF.
Conclusion
Administrative Distance enables routers to choose the most trustworthy routing information when multiple sources exist, while Autonomous Systems provide organizational structure for Internet routing. Together, these concepts ensure efficient and reliable data transmission across complex networks.
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